1. Field of the Invention
The present invention relates to a video signal encoding apparatus for compressing and encoding a video signal by dividing it into blocks and performing an orthogonal transform on each block.
2. Description of the Prior Art
If video data converted to digital signals is directly recorded on tape or other recording medium, the volume of data will be so great that it will usually exceed the limit of the data amount that the recording medium can hold. Therefore, when recording a digital video signal on tape or other recording medium, it is necessary to compress it so that the data volume does not exceed the limit. To achieve this, it has been known to compress the video signal by using a high-efficiency encoding apparatus.
One example of such high-efficiency encoding that has been widely used is the orthogonal transform encoding method in which transform coefficients obtained by orthogonal-transforming the original signal are quantized for encoding. This method is known to provide high encoding efficiency. In encoding a video signal by this method, the video signal is first divided into blocks each consisting of n.times.n pixels (where n is an integer), an orthogonal transformation is performed on each block to transform it into a transform coefficient representing n.times.n frequency regions, and then the transform coefficient is quantized. However, when all blocks are quantized with the same number of bits, adequate image quality can be obtained for the video blocks in flat areas, but noise appears in the video blocks including edge areas since errors are dispersed in the vicinity of the edge areas.
An example of an encoding apparatus that overcomes the above problem is disclosed in Japan Patent Application Laid-Open No. 2-105792. FIG. 1 shows a block diagram of the encoding apparatus disclosed n the patent Publication. The encoding apparatus shown is described below with reference to FIG. 1. A video signal is inputted to a blocking circuit 51 where it is divided into blocks, each block then being supplied to an orthogonal transfroming circuit 52 for orthogonal transformation. The transform coefficient obtained by the orthogonal transformation is quantized by a quantizing circuit 53. The quantizing circuit 53 has the ability to perform quantization using a variable number of quantization bits. An edge area detecting circuit 54 is provided to detect the edges of the video signal, while a flat area detecting circuit 55 is provided to determine whether the block represents a flat area. Based on the outputs from the edge area detecting circuit 54 and the flat area detecting circuit 55, a block identifying circuit 56 determines whether the block includes an edge area as well as a flat area, the result of which is fed to the quantizing circuit 53 to determine the number of quantization bits. When the whole block is flat or when the whole block has a complicated structure, it is decided to use a smaller bit code for quantization since noise is not appreciably visible. On the other hand, if the block includes an edge area as well as a flat area, it is decided to use a higher bit code for quantization to prevent the generation of noise in the flat area. Thus, in the encoding apparatus disclosed in the above Patent Publication, in order to overcome the aforementioned problem, the transform coefficients for blocks including both edge and flat areas are quantized using a higher bit code to reduce the noise and thereby improve the image quality after decoding. The determining factors used to detect the edge or flat areas in a block include a variance within the block, the maximum value of the block, the dynamic range of the block, etc. These factors are collectively referred to as the activity index. In the above prior art encoding apparatus, the number of quantization bits (quantization level) is selected for each block on the basis of the activity index.
The output of the quantizing circuit 53 of FIG. 1 is encoded, usually using entropy encoding such as Huffman encoding, into a variable-length code for transmission. The bit length of one block after variable-length encoding varies from block to block, and in the case of a recording medium such as a helical scan digital video tape recorder (VTR) having a fixed track length, it is convenient to grasp the number of data blocks to be recorded per track. Therefore, it is a usual practice to predetermine at least the number of data blocks to be recorded per track. Also, when block correcting codes (e.g., BCH codes, Reed-Solomon codes, etc.) are employed as error-correcting codes, it may be practiced to fix the data length of variable-length code for each error-correcting block. Usually, when encoding video signals, one field or frame is divided into N segments (where N is an integer), each segment serving as a unit, and the maximum data amount is set for each of the N units.
However, in a channel, such as a digital VTR, in which the data length for the variable-length codes is fixed, the data length of variable-length code may vary from code to code after variable-length encoding depending on the kind of the image processed, and the total code length after variable-length encoding may exceed the fixed length of the channel, resulting in an overflow. If this happens, the transmission will be cut off because of dataflow, and therefore, not only overflown data but also the subsequent data will not be transmitted. This presents the problem of an inability to correctly perform the decoding of the original signal.
Variable-length encoding of a television image is usually performed in sequence from left to right and from top to bottom of the television screen. Therefore, the problem is that the above-mentioned cutoff is likely to occur in the center of the television screen where the feature elements of the image are contained.